1. Field of the Invention
The present disclosure relates to field-effect transistors (FETs), and particularly, to nitride field-effect transistors that can be applied to power transistors.
2. Description of the Related Art
Nitride semiconductors have wider band gaps, higher breakdown electric field, and higher saturation drift velocity of electrons as compared to Si semiconductors or GaAs semiconductors. Further, in a heterostructure of AlGaN/GaN formed on a substrate whose main surface is the (0001) plane, two-dimensional electron gas (hereinafter referred to as the 2 DEG) is generated at the heterointerface by the spontaneous polarization and the piezoelectric polarization, and a sheet carrier concentration of about 1×1013 cm−2 or more is obtained even when impurities are not doped. In recent years, high electron mobility transistors (HEMTs) using this high-concentration 2 DEG as a carrier are receiving attention, and a variety of HEMT structures has been proposed (Non Patent Literature 1).
FIG. 11 is a cross-sectional view of a conventional field-effect transistor having an AlGaN/GaN heterostructure. In conventional field-effect transistor 900 shown in FIG. 11, on p-type Si substrate 901, AlN buffer layer 902, superlattice layer 903, undoped AlxGa1-xN (0≦x≦1) layer 904, undoped GaN layer 905, and undoped AlGaN layer 906 are formed in this order, and source electrode 907 and drain electrode 909 are formed on undoped AlGaN layer 906. Gate electrode 908 is formed between source electrode 907 and drain electrode 909.
In such a structure, two-dimensional electron gas 910 formed at the interface of AlGaN and GaN is generated by the spontaneous polarization and the piezoelectric polarization, and used as a carrier. When a voltage is applied between source and drain, the electrons in the channel flows from source electrode 907 to drain electrode 909. At this time, by controlling the voltage applied to gate electrode 908 to vary the thickness of the depletion layer immediately below gate electrode 908, the drain current can be controlled.